FLOOD10469

• County: Boulder
• Community: Boulder
• Stream Name: Fourmile Canyon Creek, Wonderland Creek
• Basin: South Platte
• Title: Request for LOMR - Boulder
• Floodplain - Doc Type: Correspondence

Creek just downstream of Broadway. Since the magnitude of the spills are significant
when they enter Wonderland Creek, they impact the hydrology of Wonderland Creek.

To help identify the impact of the spills on Wonderland Creek, the upstream portion of
the Fourmile Canyon Creek hydrologic model was combined with the hydrologic model
of Wonderland Creek. Rating curves of the spill discharges were developed from the
Fourmile Creek HEC-RAS model, and they were input into the hydrologic model to
simulate the spill. The model is discussed in detail in Hydrology Report For Wonderland
Creek, Boulder, Colorado (Love & Associates, Inc., 2005) which has been included in
Appendix 2 of the LOMR. In addition, please refer to the Hydraulic Analysis,
Fourmile Canyon Creek section of this report for further discussion of the hydraulic
analysis of the spill. The City and UDFCD reviewed and approved the updated
hydrology of Fourmile Canyon and Wonderland Creek.

Compared to regulatory flood peak estimates, the spill significantly decreases flood peaks
in the Fourmile Canyon Creek channel and increases flood peaks in the Wonderland
Creek channel, as shown in Table 4. The hydrologic analysis of the spill area indicates
flood peak discharges along the path of the spill from Fourmile Canyon Creek are
attenuated when they reach Wonderland Creek. The spill discharge is summarized in
Table 5.

Table 6 summarizes the location of the UDSWM nodes and HEC-RAS Cross Sections
used in the hydrologic analysis.

A copy of the hydrologic models, flow paths, and drainage area maps are provided in
Appendix 2. The hydrologic models include:

(1) CUHP (hydrographs) for Fourmile Canyon Creek and Wonderland Creek
(10-,50-, and lOO-year events);
(2) UDSWM (routing) for Fourmile Canyon Creek and Wonderland Creek, no
spill (10-, 50-, and 100-year events);
(3) UDSWM (routing) for Fourmile Canyon Creek and Wonderland Creek,
including the spill from Fourmile Canyon Creek to Wonderland Creek, (10-, 50-,
and 100-year events).

-10-